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Post subject: Re: ? on 1973 super reverb
Posted: Mon Jan 10, 2011 3:37 pm
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Why ? because we don't want to hear this "hum" in ou amp speaker.


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Post subject: Re: ? on 1973 super reverb
Posted: Mon Jan 10, 2011 4:16 pm
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Sure we don't whant to hear the hum ! 8)

But why hum is lower technically !
(lower generator impedance, pull down common mode voltage on the filament versus grilles to avoid parasitic thermo-ionic-emission ... ? these are questions not answers)

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Post subject: Re: Super Reverb
Posted: Mon Jan 10, 2011 4:38 pm
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BMW2002Ti wrote:
I glance over the AA270 and AB763 circuits of the Super Reverb & there doesn't seem to be too big a difference. The main one being that the earlier AB763 used a GZ34 tube. The AA270 called for a 5U4GB. There is also changes around the circuitry of the phase inverter. Trannies all seem to be the same part number. You may want to experiment with 5U4GB and GZ34 rectifiers, first, to see if that changes the tone more to your liking.

Just a suggestion. Good luck with that GREAT amp!

Looks like I can get a GZ34 rectifier tube for around $15.00. JD


http://www.ampwares.com/amp.asp?id=98


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Post subject: Re: ? on 1973 super reverb
Posted: Mon Jan 10, 2011 5:22 pm
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stratele52 wrote:
The two 100 resistors are there to remove the filament hum.


REALLY......?

I guess that's why there are all of those big resistors under the "doghouse" for filtering.

PFFFFFFFFFFFFFFFFFT!

Arjay

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Post subject: Re: ? on 1973 super reverb
Posted: Tue Jan 11, 2011 3:51 am
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Ok here the answer, how it work those two resistors :

Filament 6,3 volts is AC voltage and will have a 60 cycles frequency in North America. If we allow this voltage to float unreferenced to the signal reference, then an loud hum will be injected into the amp.

With a center tap on the winding, the ends of the winding are out of phase, because they are out of phase, the 60 cycles hum will be "cancelled" .

With no center tap, whe can use balancing resistor with equal values between 47 to 100 ohms. The midpoint of of this voltage divider will act like a center tap and make the two ends of the winding "look" balanced.

Sometimes one resistor is replace by a pot and labelled "hum Balance"


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Post subject: Re: ? on 1973 super reverb
Posted: Tue Jan 11, 2011 3:56 am
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I guess that's why there are all of those big resistors under the "doghouse" for filtering.

_____retroverbial


Those are, with electrolytics capacitors, for filtering the DC voltage, not the filament AC voltage.
And also for distributing right DC voltage to proper circuits.


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Post subject: Re: ? on 1973 super reverb
Posted: Tue Jan 11, 2011 4:51 am
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This may be a stupid question. Will changing the rectifier tube to GZ34 have any effect on the power tube bias? Also could changing the 15k resister on the balance pot give me a hotter bias on the power tubes. If so what size resistor should I try?


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Post subject: Re: ? on 1973 super reverb
Posted: Tue Jan 11, 2011 12:42 pm
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Changing rectifier tube change a little bit all your voltage, so you have to check your bias.

About the 15 k, if you go higher (more ohms) you'll have more negative bias voltage on the grid so tubes will run cooler.
If you put lower resistor, less neg volt.....tubes will run hot.

Witch value ? You have to make some tests.I never had do to that I always modified the pot for ajustable bias.

I'll try to schow you a picture how to modify, it's simple if you know how to solder resistors.....


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Post subject: Re: ? on 1973 super reverb
Posted: Tue Jan 11, 2011 1:58 pm
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Stratele, I think that the voltage of the filament shall be lower than the kathode. In that case the thermo-emission is from the kathode to the filament and then low injected electron hum from the filament. This is realised by the pull down of the 6.3V AC by 2 resistors.
Let me know if I'm in the right way.

Peux tu me dire ce que veut dire "doghouse" ?

8) 8) 8) 8)

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Post subject: Re: ? on 1973 super reverb
Posted: Tue Jan 11, 2011 2:10 pm
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Bonjour Tis-san,

Doghouse is funny word that somebody use to talk about the "cover" for the filter caps on top of Fender amp, You know what I mean.

The two resistor have nothing to do with lowering filament voltage. You can do a test; mesure filament voltage with and without resistors. Volt will be the same.

Bonne Année 2011 Tis-san,

Louis


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Post subject: Re: ? on 1973 super reverb
Posted: Tue Jan 11, 2011 2:13 pm
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Thanks Stratele, I need a good list of parts and diagram.


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Post subject: Re: ? on 1973 super reverb
Posted: Tue Jan 11, 2011 2:27 pm
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I come back, won't be long bluestrucker


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Post subject: Re: ? on 1973 super reverb
Posted: Tue Jan 11, 2011 2:59 pm
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Stratele,
I don't say lower the filament voltage but pull down to ground; However thanks for the information and bonne année à toi et tes proches.

Sorry Bluestrucker to disturb your discussion.

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Post subject: Re: ? on 1973 super reverb
Posted: Tue Jan 11, 2011 3:08 pm
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Tis-san wrote:
Stratele,
I don't say lower the filament voltage but pull down to ground; However thanks for the information and bonne année à toi et tes proches.

Sorry Bluestrucker to disturb your discussion.

Tis-san, No problems here!


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Post subject: Re: ? on 1973 super reverb
Posted: Tue Jan 11, 2011 3:33 pm
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You know Tis-san is not easy for me to read english, many new words when we talk amp. I apreciate what you write.

Ok bluestrucker,
I have some problem with my scanner for now.

The mods is very simple if you have the balance pot circuit I know. I explain; The pot you should have, had two (2) 10 K resistors each side (of the pot). And a wire on the center lug. And a 15 K to the third lug.

On the other end of those two resistors ( they are solder together) you have one (1) 100 K resistor;

Right ?

If your answer is yes, Just you have to do is remove those two 10 k resistors, and plug the end of the 100K to the center lug of the pot.


That's it .

After the mod is finish, on the pot, you'll have;

1- On one side the 15 K
2- On center lug : a wire and one end of the 100K
3-On the other side; a wire came from negative bias circuit.

If this is ok for you, I will write you what I suggest before power on the amp after the mod, easy.

Louis


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